1. Field of the Invention
The present invention relates to an apparatus for controlling the activity of a zinc phosphate treatment solution for forming a zinc phosphate coating on a surface of an aluminum member or an aluminum alloy member, and more particularly, it relates to an apparatus for controlling the activity of a zinc phosphate treatment solution which contains complex hydrofluoric acid as an active material.
2. Description of the Background Art
A zinc phosphate coating is formed on an aluminum material such as an aluminum member or an aluminum alloy member by a method such as that disclosed in Japanese Patent Publication No. 45-16566 (1970), for example. According to this method, a zinc phosphate coating is formed by a zinc phosphate treatment solution containing zinc ions, nitrate ions, phosphate ions, SiF.sub.6 ions, iron (II) ions and F ions as main components. When such a zinc phosphate treatment solution contains SiF.sub.6 ions and F ions as H.sub.2 SiF.sub.6 and HF, the surface of the aluminum material is etched and activated through the following chemical formulas: EQU 2Al+3H.sub.2 SiF.sub.6 .fwdarw.Al.sub.2 (SiF.sub.6).sub.3 +3H.sub.2 EQU Al+3HF.fwdarw.AlF.sub.3 +3/2H.sub.2
As understood from the above chemical formulas, HF and H.sub.2 SiF.sub.6 react with aluminum in the zinc phosphate coating treatment, to form respective aluminum salts. Consequently, free HF and H.sub.2 SiF.sub.6 concentration levels are reduced by the treatment, leading to reduction of activity levels with respect to the aluminum material. In order to control these activity levels so that the zinc phosphate treatment solution regularly has constant activity, it is necessary to measure the free HF and H.sub.2 SiF.sub.6 concentration levels.
Japanese Patent Laying-Open No. 58-211644 (1983) discloses an apparatus for controlling activity of a zinc phosphate treatment solution. This apparatus is adapted to sample a constant amount of a target solution from an electrolytic chromic acid treatment solution, add an ionic strength adjuster or the like thereto and thereafter measure fluorine concentration by an ion selective electrode method, for automatically supplying a required amount of a fluoride supplying solution when the fluorine concentration is below a target level. According to this method, complex fluorides such as H.sub.2 SiF.sub.6 and HBF.sub.4 are also measured as fluorine ions (F.sup.-). In this method, therefore, it is impossible to measure the activity of a zinc phosphate treatment solution which is mainly composed of complex hydrofluoric acid.
An apparatus which is disclosed in Japanese Patent Laying-Open No. 60-251280 (1985) is also adapted to measure fluorine concentration by a fluorine ion selective electrode method, and hence the activity of complex hydrofluoric acid cannot be measured by this apparatus.
In Japanese Patent Laying-Open No. 2-306313 (1990) which is adapted to directly measure a treatment solution stored in a treatment vessel, a metal silicon electrode meter is provided in the treatment vessel to be in contact with the solution. Also in this method, concentration of complex hydrofluoric acid cannot be measured and hence it is impossible to measure the activity of a treatment solution which is mainly composed of complex hydrofluoric acid.